Tiny Avalanche Photodiodes Target Bioterrorism Agents

In a significant finding, researchers at Northwestern University'sCenter for Quantum Devices have demonstrated solar-blind avalanchephotodiodes (APDs) that hold promise for universal biological agentdetection. Once optimized, these sensitive detectors could be combinedwith the ultraviolet light-emitting diodes (LEDs) already pioneered bythe Center for Quantum Devices to create an inexpensive detectionsystem capable of identifying the unique spectral fingerprints of abiological agent attack.

The Northwestern team, led by center director Manijeh Razeghi, becamethe first to demonstrate 280 nanometer APDs. These devices, based onaluminum gallium nitride (AlGaN) compound semiconductors, have aphotocurrent gain of more than 700.

The tiny-sized APDs should be capable of efficient detection of lightwith near single photon precision. Previously, photomultiplier tubes(PMTs) were the only available technology in the short wavelength UVportion of the spectrum capable of this sensitivity. These fragilevacuum tube devices are expensive and bulky, hindering true systemsminiaturization.

The APD technology may see further use in the deployment of systems forsecure battlefield communication. Wavelengths around 280 nanometers arereferred to as the solar-blind region; in this region, the UV light isfiltered out by the ozone layer providing for a naturally lowbackground signal. Solar-blind APDs are intrinsically able to takeadvantage of this low background level, while PMTs must use externalfilters to become solar-blind. This makes secure battlefieldcommunication possible utilizing a combination of compact, inexpensiveUV LEDs and UV APDs both developed at the Center for Quantum Devices.

The technology for the realization of solar-blind APDs is based on widebandgap AlGaN compound semiconductors. To date, no semiconductor-basedsolar-blind APDs have been reported. This is due to numerousdifficulties pertaining to the crystal growth of AlGaN compoundsemiconductors.

The major obstacle in demonstrating high performance solar-blind APDsis the high number of crystalline defects present in the AlGaNsemiconductor material. However, researchers at the Center for QuantumDevices have been able to realize high-quality AlGaN so as todemonstrate avalanche gain in the solar-blind region.

Northwestern's results were presented recently by Razeghi at the APDworkshop organized by Henryk Temkin, a new program manager at theDefense Advanced Research Projects Agency (DARPA).